Physics/speed of gravity

I read that speed of gravity is same as that of light. Is that experimentally proved? If not, why the speed of gravity cannot be measured while the speed of light can be. Please clarify.

regards,

AnswerThese guys http://www.nrao.edu/pr/2003/gravity/ have observations that put limits on the speed of the gravitational interaction at the speed of light, +/-20%. It's an observational measurement, which is largely the only way you're going to measure that. The reason is that there really is no good way to get significant mass to move at close to the speed of light, and gravitation depends on mass. If you have a postage stamp (only about 0.05 grams!) traveling at about 10% of the speed of light, you have roughly the same energy as a car going 10,000 miles per hour. Gravitation is also incredibly weak, it's a trillion*trillion*trillion times weaker (yes, multiplied) than the electromagnetic force. If you can come up with an actual good experiment to measure the speed of such a ridiculously weak force which travels so fast in the laboratory, you'd make quite a name for yourself in physics! :)

Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/CredentialsPh. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.